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UBC Theses and Dissertations

The landscape of divergence in silverleaf sunflowers Moyers, Brooke Taylor


The Texas endemic silverleaf sunflower, Helianthus argophyllus, exhibits striking genetic variation in life history: some individuals flower in late summer and are relatively short, while other individuals delay flowering in favor of growth until fall. The central goal of this research is to identify and characterize the evolutionary drivers of this variation: either local adaptation under divergent natural selection, neutral phenotypic divergence resulting from reduced gene flow and subsequent genetic drift, or both. Helianthus argophyllus exhibits strong regional genetic structure. However, populations from the central area of the species range form a single genetic cluster but are split into two phenotypic clusters: mainland coast populations, which are primarily tall and late flowering, and barrier island populations, which contain short/early flowering and tall/later flowering individuals at roughly equal frequencies. Some traits, including floral size characters, are more differentiated across the species range than is expected based on neutral genetic divergence (QST > FST), a signal of local adaptation. In a reciprocal transplant experiment, barrier island plants had higher survival rates and overall fitness than non-local individuals at barrier island sites. Observations of selection in wild populations revealed directional selection for early flowering in barrier island populations that contrasts with selection for a later flowering optimum in mainland coast populations. Collectively, these analyses support a hypothesis of adaptive divergence in flowering time in H. argophyllus, although the ecological mechanism(s) and genetic basis of this divergence have yet to be explored.

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